The present invention relates to devices for testing the accuracy of a pulse oximeter, and in particular to a device for simulating a given percentage of oxygen saturation of blood flow and heart rate which can be measured by a pulse oximeter.
In recent years, the pulse oximeter has become a useful diagnostic medical instrument for patient care which calculates the oxygen saturation of arterial blood to thereby monitor the patient""s pulmonary system. The pulse oximeter measures the amount of light absorbed by arterial hemoglobin at red and infrared waveforms and establishes a ratio between the absorption rates of the two waveforms.
The pulse oximeter is described in many documents, including U.S. Pat. No. 4,869,254 and other references. Although there are several variations in the technology for the pulse oximeters, the current technology includes a light source for generating two given wavelengths of light, typically red and infrared, which is projected through a relatively thin appendage or body portion, such as a finger or earlobe. A light detector is positioned on the opposite side of the body portion, and the intensity of the light passing through the body portion for both wavelengths are measured. The absorption of light through a given medium, such as a portion of the human body, is an exponential factor of the distance traveled and, therefore, the theory of the pulse oximeter is based upon the mathematical relationship of the Beer-Lambert law.
The pulse oximeter typically produces two wavelengths of light which are capable of penetrating the thickness of the appendage around which the light source and detector have been positioned and which have a given absorption characteristic for oxygenated and de-oxygenated hemoglobin. One typical pulse oximeter employs wavelengths of light of 660 nanometers (red) and 880 nanometers (infrared) as wavelengths which are produceable from economically available sources and which have suitable absorption characteristics of oxygenated and de-oxygenated hemoglobin. The device then calculates an oxygen saturated ratio (ROS) which fluctuates in response to changes in the permeability of the media through which the light is directed. The changes in the permeability are caused by the changes in the oxygenation of the hemoglobin. When a patient""s heart drives a pulse of oxygenated hemoglobin into the arterial structure of the appendage, the ROS for the appendage is different than when oxygenated blood is not being driven through the arterial system, and the differences of the ROS of the medium is caused solely by the presence of the oxygenated hemoglobin in the arterial system. An equation built into the device is used to calculate a measure of the oxygenation of the hemoglobin.
Since the pulse oximeter measures the light absorption for both oxygenated and unoxygenated blood, the device also calculates the patient""s pulse rate and the pulse rate is displayed as an output of the device.
The usefulness of a diagnostic instrument such as a pulse oximeter lies only in its accuracy, and if a pulse oximeter is not properly calibrated it will not provide accurate readings, and the readings will mislead medical personnel who rely on the instrument and the benefit which would otherwise be achieved by the use of the instrument will, therefore, be lost. It is necessary, therefore, to test or calibrate a pulse oximeter.
An obvious method of testing or calibrating a pulse oximeter would be to provide a simulated human appendage having characteristics which duplicate the light absorptive qualities of a living appendage through which flow pulses of the arterial blood having a given percentage of oxygenation. Such a simulator must duplicate the light absorptive qualities of an appendage of the human body between pulses of blood through the arterial system and during pulses of blood through the arterial system such that the device can thereby measure the differences of light passing through the simulated appendage and calculate the simulated oxygenation of the hemoglobin.
Prior efforts to simulate a human appendage have resulted in a mechanically operated device such as manufactured by Nonin Medical Incorporated of Plymouth, Minn. which includes an elongate member simulating the finger of a patient to which is attached a compressible bulb which can be squeezed by the operator of the test equipment. When the bulb is not being compressed, the simulated finger has the light absorptive qualities of a human finger during intervals of time between pulses of oxygenated blood in the arterial system. When the bulb is compressed, a liquid is forced within the simulated finger which alters the light absorptive qualities of the simulated finger to that of a human finger which is receiving a pulse of blood through its arterial system. Such existing devices, however, cannot test the accuracy of the pulse rate measuring capabilities of the pulse oximeter being tested. It would be desirable, therefore, to provide an arterial blood flow simulator which can accurately simulate the pulsating changes in the absorptive qualities of a human appendage in response to given pulses rates of hemoglobin having a given oxygen saturation rate passing through the appendage.
Briefly, the present invention is embodied in an artificial blood flow simulator to be used to test or calibrate a pulse oximeter. Briefly, the arterial blood flow simulator has a body comprising material which is at least partially transparent to red and infrared light waves. The body is shaped and sized like that of a human appendage which can be received between the light sources and the light sensors of a pulse oximeter. Within the body is a light valve which is responsive to an electronic signal for varying the amount of light passing through the body. Connected to the light valve is a signal generator for generating a pulsating electronic signal which corresponds to a given blood flow amplitude, that is, corresponds to blood flow having a given pulse rate and a given oxygenation of the hemoglobin in the arterial blood flow.
The device will have the permeability of a human appendage between pulses of blood in the arterial system when the signal generator is generating an electronic pulse and will have the permeability of an appendage having arterial blood flowing therethrough when the signal generator is not generating an electronic pulse.